15.6 is a Darwin kernel version from 2018. It's the number `uname -a` reports. We should probably just switch to using XNU version numbers, which are in the 10000s now, so there's no confusion. I'm reasonably certain it works that far back, but I currently lack the ability to spin up old MacOS VMs for testing. Caveat emptor anyone not running MacOS on a recent version.

This is jart we are talking about. Perhaps, having made code Actually Portable in space, now she is doing time.